So I think I may have lost my rear diff. I tried to run my Yokomo today and the back wheels wouldnt spin from take off at all...just the front spin. Then when I hold the car up and get all the wheels to spin and then put the car back on the ground the rear wheels die. I've opened up the rear gearbox and took a look at the diff and this is what it looks like:








If its beyond repair should I just lock the diff? If I should lock it how do you do that? Or should i just get a new diff?

could be a number of things,
your screw may have come loose. in that case tighten it
the nut or screw could have stripped, if so replace the stripped part
or your diff may have melted in which case u need new outdrives and to build a new diff with the parts.

Yeah I was looking at the manual and I noticed that theres a screw to tighten it. Now its locked but it works!